Astrophysical mechanisms for generating X-rays

There are three radiation processes – thermal, synchrotron and blackbody – that are the dominant mechanisms for producing X-rays in an astronomical setting, and whenever high-energy electrons are present, we must add inverse Compton scattering of microwave background photons into the X-ray regime. The spectral signature of each process is unique and is therefore one of the first clues to the nature of an unknown X-ray source. If the spectrum can be measured with high resolution over a broad energy band, then usually both the emission process and the physical conditions within the source can be deduced.

Thermal emission from a hot gas

Consider a hot gas of low enough density that it can be described as thin and transparent to its own radiation. This is not difficult to achieve for X-rays. At temperatures above 105 K, atoms are ionised, and a gas consists of positive ions and negative electrons. Thermal energy is shared among these particles and is transferred rapidly from one particle to another through collisions. Indeed thermal equilibrium means that the average energy of all particles is the same and is determined only by the temperature. When an electron passes close to a positive ion, the strong electric force causes its trajectory to change. The acceleration of the electron in such a collision causes it to radiate electromagnetic energy, and this radiation is called bremsstrahlung (literally, ‘braking radiation’).